1. Field of the Invention
The present invention relates to olefin polymerisation. More specifically, the present invention concerns a process for continuously polymerising olefins, such as ethylene and/or propylene, optionally together with C4 to C12 alpha-olefins, in slurry phase in the presence of a polymerisation catalyst. The invention also concerns a method of continuously withdrawing polymer slurry from the reactor and concentrating the slurry.
2. Description of Related Art
Slurry processes for polymerising olefins are known since the development of Ziegler catalysts in 1950's. The particle form process of Phillips, where ethylene was polymerised in the presence of a chromium catalyst in a loop reactor at elevated pressure and temperature, was developed at the same time.
It is common to concentrate the slurry that is or has been withdrawn from the reactor. Earlier, this was done by withdrawing the slurry via settling legs. These are vertical pipes where the slurry is allowed to settle. When a valve connected to the lower end of the settling leg is opened, the settled slurry is withdrawn from the reactor. The valve is then closed and a new portion of slurry is allowed to settle. The disadvantage of the settling legs is that they operate intermittently and lead to a pulsating product flow.
It is also known that hydrocyclones can be used for concentrating the slurry withdrawn from the reactor. Such processes are disclosed, e.g., in U.S. Pat. No. 3,816,383, U.S. Pat. No. 4,395,523 and EP 1 118 624. The particular advantage of hydrocyclones is that they can be used in a truly continuous manner. Although hydrocyclones are mechanically relatively simple, the high pressure employed and reactivity of the slurries make instrumentation and piping around the hydrocyclones somewhat complicated, which increases investment costs.
There is therefore still a need for a process, which makes it possible to concentrate the polymer slurry withdrawn from the reactor in a simple and economic way.